Memory devices, memory device constructions, constructions, memory device forming methods, current conducting devices, and memory cell programming methods

ABSTRACT

Some embodiments include memory devices having a wordline, a bitline, a memory element selectively configurable in one of three or more different resistive states, and a diode configured to allow a current to flow from the wordline through the memory element to the bitline responsive to a voltage being applied across the wordline and the bitline and to decrease the current if the voltage is increased or decreased. Some embodiments include memory devices having a wordline, a bitline, memory element selectively configurable in one of two or more different resistive states, a first diode configured to inhibit a first current from flowing from the bitline to the wordline responsive to a first voltage, and a second diode comprising a dielectric material and configured to allow a second current to flow from the wordline to the bitline responsive to a second voltage.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional and claims priority of U.S. patentapplication Ser. No. 12/125,797, which was filed on May 22, 2008 nowU.S. Pat. No. 8,120,951, and which is incorporated herein by reference.

TECHNICAL FIELD

Memory devices, memory device constructions, constructions, memorydevice forming methods, current conducting devices, and memory cellprogramming methods.

BACKGROUND

A continuing goal of integrated circuit fabrication is to decrease theamount of semiconductor real estate consumed by integrated circuitdevices, and to thereby increase the level of integration.

Memory may utilize a large array of memory devices. Accordingly,reduction in the size of individual memory devices may translate into alarge increase in the bit density. Common memory devices are dynamicrandom access memory (DRAM) devices, static random access memory (SRAM)devices, and nonvolatile devices (so-called flash devices). Thenonvolatile devices may be incorporated into NAND or NOR memory arrayarchitectures.

The size of a memory device may be expressed in terms of the smallestfeature size utilized in fabrication of the memory device. Specifically,if the smallest feature size is designated as “F,” the memory devicedimensions may be expressed in units of F². Conventional DRAM memoryfrequently comprises dimensions of at least 6F², and SRAM may requireeven more semiconductor real estate.

A type of memory that potentially consumes very little semiconductorreal estate is so-called cross-point memory. In cross-point memory, amemory cell occurs at overlap between a wordline and a bitline.Specifically, a material which undergoes a stable and detectable changeupon exposure to current is provided between the wordline and bitline.The material may be, for example, a perovskite material, a chalcogenidematerial, an ionic transport material, a resistive switching material, apolymeric material and/or a phase change material. Since the memory cellmay be confined to a region of overlap of a bitline and wordline, thememory cell may theoretically be formed to dimensions of 4F² or less.

Problems encountered in closely packing cross-point memory may includedisturbance mechanisms (or so-called cross-talk) occurring when datatransfer to or from one memory cell influences a neighboring memorycell.

It is desired to develop improved methods for forming highly integratedcircuitry, and to develop improved highly integrated circuitconstructions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross-sectional view of a portion of aconstruction in accordance with an embodiment.

FIG. 2 is a diagrammatic cross-sectional view of a portion of aconstruction in accordance with an embodiment.

FIG. 3 shows three band-gap diagrams illustrating three different biasconditions of a diode in accordance with an embodiment.

FIG. 4 is a diagrammatic cross-sectional view of a portion of aconstruction in accordance with an embodiment. FIG. 4 also shows aschematic electrical diagram of some of the components of thecross-section.

FIG. 5 is a schematic electrical diagram of an array of memory elementsin accordance with an embodiment.

FIG. 6 is a chart depicting a voltage probability distribution functionin accordance with an embodiment.

FIG. 7A is a chart depicting a voltage-current relationship inaccordance with an embodiment.

FIG. 7B is a chart depicting current probability distribution functionsin accordance with an embodiment.

FIG. 7C is a chart depicting a current probability distribution functionin accordance with an embodiment.

FIG. 8A is a chart depicting a voltage-current relationship inaccordance with an embodiment.

FIG. 8B is a chart depicting current probability distribution functionsin accordance with an embodiment.

FIG. 8C is a chart depicting a current probability distribution functionin accordance with an embodiment.

FIG. 9 is a chart depicting a voltage-current relationship in accordancewith an embodiment.

FIG. 10 is a diagrammatic cross-sectional view of a portion of aconstruction in accordance with an embodiment. FIG. 10 also shows aschematic electrical diagram of some of the components of thecross-section.

FIG. 11 is a schematic electrical diagram of an array of memory elementsin accordance with an embodiment.

FIG. 12 is a diagrammatic cross-sectional view of a portion of aconstruction in accordance with an embodiment. FIG. 12 also shows aschematic electrical diagram of some of the components of thecross-section.

FIG. 13 is a diagrammatic cross-sectional view of a portion of aconstruction in accordance with an embodiment. FIG. 13 also shows aschematic electrical diagram of some of the components of thecross-section.

FIG. 14 is a diagrammatic cross-sectional view of a portion of aconstruction in accordance with an embodiment. FIG. 14 also shows aschematic electrical diagram of some of the components of thecross-section.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

In some embodiments, cross-point memory cells are formed to includediodes. The diodes may be configured to enable current to pass to orfrom a portion of the memory cell, while also alleviating, and possiblypreventing, cross-talk between adjacent devices. The diodes may containstacked thin dielectric films, with the dielectric films beingband-structure engineered to achieve tailored diode properties forparticular memory cells.

It may be advantageous to utilize stacked dielectric materials for thediodes rather than utilizing conventional silicon-based n-p junctiondiodes. The conventional silicon-based junction diodes may be limitedrelative to bandgap, Shockley-Read-Hall (SRH) generation andrecombination rates, active doping concentrations, injection velocity,carrier lifetime, and breakdown strength (or other high field propertiessuch as ionization rates, etc.).

The cross-point memory cells may be arranged in vertical stacks.Stacking of the memory cells may substantially reduce the real estateconsumption attributed to individual memory cells. For instance, if two4F² memory cells are stacked such that one is directly over the other,then the amount of semiconductor real estate consumed by each iseffectively cut in half so that the individual memory cells areessentially only consuming 2F² of semiconductor real estate. Thereduction in effective real estate consumption increases proportionallyto the number of vertically-stacked memory cells. Thus, significantstrides in integration may be achieved by vertically stacking at leastsome of the memory cells of a memory cell array.

The stacked memory cells may be utilized as nonvolatile memory, and maycorrespond to single level cells (SLCs) or multilevel cells (MLCs). Suchnonvolatile memory may be incorporated into NAND memory arrays. Inembodiments in which multi-stacked multilevel cells (MS-MLCs) areformed, the memory may prove to be particularly low-cost,high-performance, and high-density. The stacked cells may be routedthrough multi-level interconnects.

In some embodiments, the fabrication of memory cells is conducted over asilicon substrate utilizing low-temperature deposition processes, andwith few if any high temperature dopant activation steps. Avoidance ofhigh-temperature processing may alleviate thermally-induced damage tointegrated circuit devices. Also, many of the materials showing promisefor utilization as memory elements in cross-point memory cells (forinstance, Ge₂Se₂Te₅ and other chalcogenides, various metal oxides, etc.)lack high-temperature stability.

Example embodiments are described with reference to FIGS. 1-14.

Referring to FIG. 1, a fragment of a diode construction 2 isillustrated. The fragment comprises a base 12 and a diode 26 over base12.

Base 12 may comprise semiconductor material, and in some embodiments maycomprise, consist essentially of, or consist of monocrystalline silicon.The base may be referred to as a semiconductor substrate. The terms“semiconductive substrate,” “semiconductor construction” and“semiconductor substrate” mean any construction comprisingsemiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials), and semiconductive materiallayers (either alone or in assemblies comprising other materials). Theterm “substrate” refers to any supporting structure, including, but notlimited to, the semiconductive substrates described above.

Although base 12 is shown to be homogenous, it may comprise numerouslayers in some embodiments. For instance, base 12 may correspond to asemiconductor substrate containing one or more layers associated withintegrated circuit fabrication. In such embodiments, such layers maycorrespond to one or more of metal interconnect layers, barrier layers,diffusion layers, insulator layers, etc. In some embodiments, anuppermost region of the base may comprise an electrically insulativematerial so that a conductive layer of diode 26 is directly against suchinsulative material. In some embodiments, base 12 may comprise asemiconductor-on-insulator (SOI) construction.

Diode 26 comprises conductive materials 22 and 32 and insulativematerial 34. In some embodiments, conductive materials 22 and 32 may bereferred to as conductive diode materials (or in other words, as diodeelectrodes). Conductive materials 22 and 32 may comprise any suitablecomposition or combination of compositions, and may, for example,comprise, consist essentially of, or consist of one or more of variousmetals (for instance, tantalum, platinum, tungsten, aluminum, copper,gold, nickel, titanium, molybdenum, etc.), metal-containing compositions(for instance, metal nitrides, metal silicides such as tungsten silicateor tantalum silicide, etc.), and conductively-doped semiconductormaterials (for instance, conductively-doped silicon). In someembodiments, conductive materials 22 and 32 may each have a thickness offrom about 2 nanometers to about 20 nanometers.

In some embodiments, material 22 may include one of aluminum, tungsten,molybdenum, platinum, nickel, tantalum, copper, titanium, tungstensilicide, or tantalum silicide and material 32 may include a differentone of aluminum, tungsten, molybdenum, platinum, nickel, tantalum,copper, titanium, tungsten silicide, or tantalum silicide.

Insulative material 34 may be referred to as diode dielectric material,and may comprise any suitable composition or combination ofcompositions. As illustrated by FIG. 1, insulative material 34 may be indirect physical contact with both material 22 and material 34.

In some embodiments, insulative material 34 comprises a stack ofelectrically insulative layers, with the individual layers having bandgap and/or band-alignment properties tailored for the particularapplication of the diode. The layers may have individual thicknesses offrom about 0.7 nanometers to about 5 nanometers and may comprise,consist essentially of, or consist of one or more compositions selectedfrom the group consisting of aluminum nitride, aluminum oxide, hafniumoxide, magnesium oxide, niobium oxide, silicon nitride, silicon oxide,tantalum oxide, titanium oxide, yittrium oxide, and zirconium oxide. Theoxide and nitrides are referred to in terms of the principal components,rather than in terms of specific stoichiometries. Accordingly, the oxideof silicon is referred to as silicon oxide, which encompasses thestoichiometry of silicon dioxide.

Diode 26 may be configured to conduct current from material 32 tomaterial 22 when a first voltage is applied across material 32 andmaterial 22 with material 32 being at a higher potential than material22. Diode 26 may also be configured to inhibit current from flowing frommaterial 22 to material 34 when a second voltage is applied acrossmaterial 32 and material 22 with material 22 being at a higher potentialthan material 32. Accordingly, the second voltage may have a polarityopposite that of a polarity of the first voltage. In some embodiments,the first voltage may be between about 0.5 volts and 1.5 volts and thesecond voltage may be between about 0 volts and −15 volts. Accordingly,diode 26 may be characterized as a selectively conducting device whoseability to conduct current depends on an applied bias voltage.

In some embodiments, the first voltage may have the same magnitude asthe second voltage. Accordingly, diode 26 may allow current to flow frommaterial 32 to material 22 when forward biased with a voltage but mayinhibit current from flowing from material 22 to material 32 whenreverse biased with the same voltage.

Tunneling properties of material 34, and/or carrier injection propertiesof conductive materials 22 and 32, may be tailored to engineer desiredproperties into diode 26. For example, materials 22, 32, and 34 may beengineered so that diode 26 allows electrons to tunnel from material 22through material 34 to material 32 when the first voltage describedabove is applied across material 32 and material 22 but inhibitselectrons from tunneling from material 32 to material 22 when the secondvoltage described above is applied across material 32 and material 22.

Referring to FIG. 2, a fragment of a diode construction 4 isillustrated. Similar numbering is used in referring to FIG. 2 as is usedabove in describing FIG. 1 where appropriate. Fragment 4 depicts anotherembodiment of diode 26. Fragment 4 includes base 12 and diode 26 overbase 12.

In the shown embodiment, diode dielectric material 34 comprises a stackof three different dielectric materials 54, 56, and 58. Such materialsmay be tailored relative to one another so that band gaps, and/orconduction band edges, and/or valence band edges, between the materialsenable tunneling of carriers in one direction through the materials, butnot in an opposing direction.

The dielectric materials 54, 56, and 58 may comprise any suitablematerials, and may, for example, comprise one or more compositionsselected from the group consisting of aluminum nitride, aluminum oxide,hafnium oxide, magnesium oxide, niobium oxide, silicon nitride, siliconoxide, tantalum oxide, titanium oxide, yittrium oxide, and zirconiumoxide.

Although example diode 26 of FIG. 2 has three different dielectricmaterials (54, 56, and 58), in other embodiments diode 26 may compriseother than three different dielectric materials. Specifically, in someembodiments diode 26 may comprise more than three different dielectricmaterials, and in other embodiments, diode 26 may comprise less thanthree different dielectric materials. The number of different dielectricmaterials used in diode 26 may affect the speed with which the diodereacts to a voltage. For example, as the number of different dielectricmaterials increases, a difference between a time when diode 26 is biasedwith a voltage and a time when current begins to flow through diode 26responsive to the voltage may decrease. However, as the number ofdifferent dielectric materials increases, a magnitude of a voltage usedto forward bias diode 26 may also increase.

The embodiment of FIG. 2 may be fabricated as follows. Initially,material 22 may be formed over base 12. Material 22 may be patterned byutilizing photolithographic processing and one or more etches.Subsequently, material 54 may be formed over material 22. In someembodiments, material 54 may be deposited on material 22 and may bepatterned using photolithographic processing and one or more etches.Material 54 may be deposited with any suitable methodology, including,for example, atomic layer deposition (ALD). Materials 56 and 58 maysubsequently be deposited over material 54 using one or more of thetechniques described above in relation to material 54.

In some embodiments, the methods used in forming materials 54, 56, and58 may be selected so that the methods do not substantially change thedimensions of material 22 or otherwise render material 22 inoperable asan electrode of diode 26. For example, a maximum temperature used informing materials 54, 56, and 58 may be below a melting temperature ofmaterial 22 so that material 22 does not change dimension or shape as aresult of the formation of materials 54, 56, and 58. By way of anotherexample, materials 54, 56, and 58 may be undoped. Accordingly, annealingmight not be used in forming materials 54, 56, and 58. Forming thesematerials without annealing may be advantageous because annealing mayinvolve undesirably altering dimensions of material 22 as a result ofhigh temperatures used during annealing.

Subsequently, material 32 may be formed over material 58. Material 32may be patterned by utilizing photolithographic processing and one ormore etches. Material 32 may be undoped and the formation of material 32might not use a temperature higher than a melting temperature ofmaterial 22.

FIG. 3 shows band gap diagrams of diode 26 in an unbiased condition(diagram 60), a forward biased condition (diagram 62) and a reversebiased condition (diagram 64). Diagrams 60 and 64 show that in anunbiased condition, and in a reverse biased condition, bands fromdielectric materials 58, 56, and 54 preclude migration of carriersbetween conductive materials 22 and 32. In contrast, diagram 62 showsthat tunneling may occur in a forward biased condition so that carriers(specifically electrons in the shown embodiment) may tunnel fromconductive material 22 to conductive material 32 via quantum wells 66.The flow of the carriers is diagrammatically illustrated with a dashedarrow 63 in FIG. 3. It is noted that the diodes shown in FIGS. 1 and 2are oriented for current flow from conductive material 32 to conductivematerial 22. Such is consistent with the diagrams of FIG. 3 whichillustrate electron flow from conductive material 22 to conductivematerial 32 (in other words, in an opposite direction to the currentflow). In other embodiments, the arrangement of material 54, 56, and 58may be reversed so that the electron flow in the forward-biasedcondition is from conductive material 32 to conductive material 22.

The band structures of FIG. 3 may be considered engineered bandstructures. Heterostructures may be formed by molecular beam epitaxy(MBE) growth of III/V materials. In dielectric materials, a band gap maybe engineered through thermal treatments (such as thermal treatment ofaluminum oxides), as is known for nonvolatile memory cells (such as“crested barrier” cells and VARIOT flash cells). The band gap engineeredstructures may exploit characteristics of band-edge discontinuities incarrier transport in the semiconductor, and/or may exploitcharacteristics of band-edge discontinuities in charge storage of thedielectric. For nonvolatile memory cells, this may enable optimizationof retention, and endurance characteristics.

Deposition of thin layers of dielectric material may create localquantum wells 66 which may be exploited in the diode structuresdescribed herein. The conduction band and valence band edges of thedielectrics may be engineered by material choice and/or thermaltreatments. Fermi-level pinning in the metal region may be engineered bytailoring the compositions of the conductive materials at the tops andbottoms of the diodes. The barrier heights along the dielectricthickness may determine the tunneling characteristics of the structures.

The diodes described in FIGS. 1 and 2 may be considered band-gapengineered in that compositions of materials 22, 32, 54, 56, and 58 arechosen so that the forward-biased tunneling of diagram 62 occurs. Inchoosing materials 22 and 32, work functions may be considered. A workfunction may be related to an amount of energy used to remove anelectron from a metal. In FIG. 3, heights of the bars corresponding tomaterials 22 and 32 may represent work functions of materials 22 and 32.As illustrated in FIG. 3, material 22 may have a higher work function(represented by a higher bar) than material 32. Consequently, an amountof energy used to remove an electron from material 22 may be larger thanan amount of energy used to remove an electron from material 32.Designing material 22 to have a higher work function than material 32may help enable electrons to tunnel from material 22 through materials54, 56, and 58 to material 32.

In choosing materials 54, 56, and 58, barrier heights may be considered.A barrier height may be related to an energy difference between aconduction band of a material and a valence band of the material. InFIG. 3, heights of the bars corresponding to materials 54, 56, and 58may represent barrier heights of materials 54, 56, and 58. In someembodiments, barrier heights of materials 54, 56, and 58 may be greaterthan work functions of materials 22 and 32 as is illustrated by diagram60 of FIG. 3.

Band-gap engineering diode 26 may include selecting materials 54, 56,and 58 so that barrier heights of materials 54, 56, and 58 have aparticular relationship. For example, each of materials 54, 56, and 58may have a different barrier height. Further, as illustrated in diagram60, materials 54, 56, and 58 may be arranged between materials 22 and 32in order of increasing barrier height. Accordingly, material 54 (whichis closest to material 22) may have the lowest barrier height ofmaterials 54, 56, and 58, material 56 may have a barrier height largerthan material 54, and material 58 may have a barrier height larger thanmaterial 56.

Materials 54, 56, and 58 may be chosen to have valence band energylevels that are aligned with respect to one another. By way of example,the valence band energy levels of materials 54, 56, and 58 may bealigned if the valence band energy levels of materials 54, 56, and 58are substantially the same. Alternatively, materials 54, 56, and 58 maybe chosen to have conduction band energy levels that are aligned withrespect to one another. By way of example, the conduction band energylevels of materials 54, 56, and 58 may be aligned if the conduction bandenergy levels of materials 54, 56, and 58 are substantially the same.

Materials 54, 56, and 58 may be selected so that quantum wells 66 arecreated at the junction between material 54 and material 56 and at thejunction between material 56 and 58 when diode 26 is forward biased. Aswas described above, diode 26 may be forward biased by a voltage appliedacross materials 32 and 22 so that material 32 is at a higher potentialthan material 22. Furthermore, in a forward biased condition, quantumwells might form between conductive materials at the tops and bottoms ofthe diodes (with such conductive materials being the electrodes of thediodes).

In some embodiments, during formation of diode 26, properties ofmaterials 54, 56, and 58 may change slightly at the interfaces betweenmaterials 54, 56, and 58 due to processing steps undertaken to formdiode 26. For example, small portions of materials 54 and 56 at theinterface between materials 54 and 56 may mix with each other duringprocessing, even at relatively low processing temperatures less than 550degrees C. Mixing of portions of materials 54 and 56 may degrade theabrupt change in barrier height between materials 54 and 56 illustratedin FIG. 3, which may affect the formation of a quantum well betweenmaterials 54 and 56.

To inhibit mixing of portions of materials 54 and 56 and mixing ofportions of materials 56 and 58, a first very thin layer of insulatingmaterial may be formed between materials 54 and 56. The first very thininsulating layer may be a monolayer and may prevent mixing betweenmaterials 54 and 56, thereby preserving the abrupt change in barrierheight between materials 54 and 56. When diode 26 is forward biased,carriers may move freely through the first very thin insulating layerdue to the carrier tunneling effect described herein. A similar secondvery thin insulating layer may be formed between materials 56 and 58 toprevent mixing between portions of material 56 and portions of material58. The first and/or second very thin insulating layers may be formed,for example, by deposition using ALD. Alternatively, the first and/orsecond very thin insulating layers may be formed by passivation ofdielectrics and/or anneals, such as in a nitrogen ambient.

The quantum wells will have discrete energy levels. The contact betweenone electrode and an adjacent dielectric will have a first Fermi level.When energy is provided the state may be raised to a first allowedquantum energy level, which may dramatically increase the probability ofcarrier tunneling. This may lead to an effective lowering of thepotential barrier in the dielectric.

In a reverse bias condition (such as the condition depicted by diagram64), the potential barrier is high and formation of any quantum well issuppressed. There is, therefore, a low probability for conductioncurrent to flow from one metal to another—due to reduced tunneling,which approaches zero—if the dielectric thickness is appropriatelytailored.

Tunneling characteristics across structures such as diode 26 indicatethat there may be a sharp turn-on characteristic when the Fermi levelcorresponds to a lowest allowed quantum energy level. The results may bemodified in the presence of phonons at higher temperatures, but anon-linear characteristic may result from such structure.

Tunneling may be a very fast process, and may occur in femto seconds.Tunneling may also be relatively independent of temperature.Accordingly, thin film diodes of the type described herein may be ableto be switched very fast, and to meet high temperature reliabilitycriteria. For example, diode 26 may be forward biased and current mayflow through diode 26. Diode 26 may subsequently be reverse biased sothat current is inhibited from flowing through diode 26. Diode 26 may berepeatedly forward biased and then reverse biased in this manner at ahigh rate. In some embodiment, the rate may exceed 10 Ghz.

Some example compositions suitable for the band-gap engineered diodesare aluminum for material 22, aluminum oxide for material 58, silicondioxide for material 56, silicon nitride for material 54, and tungstenfor material 32. Another set of exemplary compositions is molybdenum formaterial 22, silicon dioxide for material 58, silicon nitride formaterial 56, hafnium oxide for material 54 and platinum for material 32.Another set of exemplary compositions is platinum for material 22,silicon dioxide for material 58, hafnium oxide for material 56,zirconium oxide for material 54 and nickel for material 32.

Referring to FIG. 4, a fragment of a construction 6 is illustrated.Similar numbering is used in referring to FIG. 4 as is used above indescribing FIGS. 1-3 where appropriate. The fragment comprises a base 12and a memory cell 10 over the base.

A schematic electrical diagram 8 is shown adjacent fragment 6 toillustrate some of the electrical components of the fragment. Theelectrical diagram shows that memory cell 10 comprises a bitline 22, awordline 24, a diode 26 and a memory element 28.

Wordline 24 and bitline 22 are shown in construction 6 to compriseelectrically conductive material. Such electrically conductive materialmay comprise any suitable composition or combination of compositions,including one or more of various metals (for instance, tantalum,platinum, tungsten, aluminum, copper, gold, etc.), metal-containingcompositions (for instance, metal nitrides, metal silicides, etc.), andconductively-doped semiconductor materials (for instance,conductively-doped silicon). The individual wordlines and bitlines mayhave thicknesses of from about 2 nanometers to about 20 nanometers.

Memory elements 28, conductive material 32, and wordline 24 togetherform memory component 35. Memory element 28 may comprise any suitablecomposition or combination of compositions, and may, for example,comprise, consist essentially of, or consist of one or more ofperovskite materials, chalcogenide materials, ionic transport materials,resistive switching materials, polymeric materials and phase changematerials.

Current within one or both of the conductive materials 32 and 24 ofmemory component 35 may be utilized to change a state of memory element28 in a programming operation, or to ascertain a state of memory element28 in a reading operation. In some embodiments, after a current has beenused to change a state of memory element 28 in a programming operationand the current has ceased, memory element 28 may remain in the newstate in the absence of a current or a voltage.

Conductive material 32, insulative material 34, and bitline 22 togetherform diode 26 as was described in detail above in relation to FIGS. 1-3.Conductive material 32 is overlapped by memory component 35 and diode26. In some embodiments, conductive material 32 may be referred to asconductive diode material (or in other words, as a diode electrode),even though material 32 is also part of memory component 35.

In schematic electrical diagram 8, diode 26 is shown between bitline 22and memory component 35. In other embodiments, diode 26 may beadditionally or alternatively provided between wordline 24 and memoryelement 28.

In the shown embodiment, diode 26 permits current flow from memorycomponent 35 to bitline 22, but restricts current flow in the opposingdirection. Such can enable reading to and writing from individual memoryelements, while limiting cross-talk between adjacent memory elements.

Although the diode 26 is shown oriented to direct current flow frommemory component 35 to bitline 22, in other embodiments the orientationof diode 26 may be reversed. Accordingly, diode 26 may be oriented topermit current flow from bitline 22 to memory component 35, and torestrict current flow in the opposing direction.

In some embodiments, memory cell 10 may be incorporated into an arraycomprising both vertical stacking of memory cells and horizontalarrangement of memory cells. In some embodiments, wordline 24 may bepart of a plurality of memory cells (e.g., a column of memory cells) inaddition to memory cell 10 and may extend substantially orthogonally tobitline 22. Bitline 22 may be part of plurality of memory cells (e.g., arow of memory cells) in addition to memory cell 10. The term“substantially orthogonally” means that the bitlines and wordlines aremore orthogonal to one another than not, which can include, but is notlimited to, embodiments in which the wordlines and bitlines are entirelyexactly orthogonal to one another.

FIG. 5 is a schematic circuit diagram illustrating one embodiment of anarray of memory cells. FIG. 5 shows memory elements 28 and diodes 26between wordlines 24 and bitlines 22, and further shows diodes 26connected between memory elements 28 and bitlines 22.

The embodiment of FIG. 5 may be fabricated as follows. Initially,bitlines 22 may be formed over semiconductor base (or substrate) 12.Bitlines 22 may be patterned utilizing photolithographic processing andone or more etches to pattern bitline material into a plurality oflines.

Subsequently, a first level of diode dielectric material 34 (which maybe a stack of multiple dielectric layers, e.g., as discussed above inrelation to FIGS. 1-3) is formed over the bitlines. Diode dielectricmaterial 34 may be deposited across the bitlines and spaces between thebitlines, and then patterned utilizing photolithographic processing andone or more etches to create the configuration shown in FIG. 4. In someembodiments, diode dielectric material 34 is only at cross-points of thewordlines and bitlines. In some embodiments, the diode dielectricmaterial may be left between the bitlines rather than patterned to beonly at cross-points of the wordlines and bitlines. Diode dielectricmaterial 34 may be deposited with any suitable methodology, including,for example, ALD.

A first level of electrically conductive diode material (i.e., a diodeelectrode) 32 is then formed over diode dielectric material 34.Electrically conductive material 32 may be formed in the configurationshown in FIG. 4 by depositing the material and then patterning it with aphotolithographically patterned mask and one or more etches.

Memory elements 28 are then formed over electrically conductive material32. The memory elements may be formed by depositing memory elementmaterial across the bitlines and spaces between the bitlines, and thenpatterning the memory element material utilizing photolithographicprocessing and one or more etches to create the shown configuration inwhich the memory element material is only at cross-points of thewordlines and bitlines. In some embodiments, the memory element materialmay be left between the bitlines rather than patterned to be only atcross-points of the wordlines and bitlines.

A first level of wordline material is formed over the memory elements.The wordline material may be deposited across the bitlines and spacesbetween the bitlines, and then patterned utilizing photolithographicprocessing and one or more etches to create the shown configuration inwhich the bitlines are substantially orthogonal to the wordlines.

Subsequent levels of bitlines, diode dielectric, conductive diodematerial, memory elements, and wordlines may be formed using subsequentiterations of the above-discussed processing, separated by a passivationlayer, to form vertically stacked memory arrays to desired heights. Insome embodiments, the vertical stacks may comprise at least 3 memorycells, at least 10 memory cells, or at least 15 memory cells.

The vertically stacked memory cells may be identical to one another, ormay differ from one another. For instance, the diode material utilizedfor memory cells at one level of a vertical stack may be different incomposition from the diode material utilized for memory cells at anotherlevel of a vertical stack; or may be the same composition as the diodematerial utilized for the memory cells at the other level of thevertical stack.

FIG. 4 illustrates diode 26 provided between the bitline 22 and memorycomponent 35. In other configurations, including configurations in whichmemory cells are stacked as discussed above, diode 26 may be providedbetween memory component 35 and wordline 24. The fabrication processutilized to form such other configurations may be similar to thatutilized to form the configuration of FIG. 4, except that the conductivediode material and diode dielectric material may be formed afterformation of the memory elements rather than before formation of thememory elements. In yet other embodiments, the orientation of thewordlines and bitlines in the memory cells may be reversed (so that thewordlines are under the bitlines) and the diodes may be formed eitherbetween the wordlines and the memory elements, or between the bitlinesand the memory elements.

Returning now to FIG. 5, in order to determine a resistive state of aselected one of the memory cells of FIG. 5, a read voltage may beapplied across a selected one of the wordlines 24 and a selected one ofthe bitlines 22. In response, a current may flow from the selectedwordline 24 to the selected bitline 22. This current may be measured todetermine a resistive state of the selected memory cell. For example, ifthe selected memory cell is configured to be programmed in either a highresistance state or a low resistance state, the current may be measuredto determine whether the current corresponds to the high resistancestate or the low resistance state. In some embodiments, the memory cellmay be used to store a single bit of information with the low resistancestate corresponding to a bit value of “0” and the high resistance statecorresponding to a bit value of “1.”

Each of the memory cells of FIG. 5 may be similarly read using a readvoltage as described above by applying the read voltage to the wordlineand bitline corresponding to the memory cell to be read. Ideally, amagnitude of the read voltage will be the same regardless of the memorycell being read. However, in some embodiments, a read voltage applied toone memory cell may have a larger or smaller magnitude than a readvoltage applied to another memory cell. The difference in read voltagesmay result from one or more of a number of different factors. Forexample, the read voltage may vary slightly depending on the position ofthe memory cell within the array of memory cells due to the lengths ofthe wordlines and/or bitlines. The difference in read voltages appliedto the memory cells may be described statistically by a probabilitydensity function (PDF).

FIG. 6 illustrates such a PDF. As depicted in FIG. 6, the read voltageapplied most often between a wordline and bitline may be “V.” However,as indicated by the probability density function, there is a significantprobability that the read voltage may be higher or lower than “V.” Insome embodiments, the probability density function may be a normal orGaussian distribution.

FIG. 7A illustrates one embodiment of a relationship between voltagesapplied across a memory cell and currents conducted by the memory cellin response to the voltages. As depicted in FIG. 7A, as the voltageacross the memory cell increases, the current increases. In someembodiments, the relationship between voltage and current may besubstantially linear.

In some embodiments, memory element 28 may be configured in one of fourdifferent resistive states. Accordingly, memory element 28 may representtwo bits of information. Of course, other embodiments are possible inwhich memory element 28 has more or fewer than four different resistivestates and therefore represents more or fewer than two bits ofinformation. When a voltage (e.g., a read voltage) is applied acrossmemory cell 10, an amount of current that memory cell 10 conducts inresponse to the voltage may depend on the resistive state of memoryelement 28. Thus, the present resistive state of memory element 28 maybe determined by measuring a current conducted by memory cell 10 inresponse to the voltage. As was discussed above, the voltage appliedacross memory cell 10 may vary according to a PDF such as the PDFillustrated in FIG. 6.

FIG. 7B illustrates four current probability density functions 14, 16,18, and 20 superimposed on each other. PDF 14 may represent theprobability that memory cell 10 will conduct a particular amount ofcurrent when a voltage (such as a read voltage) having the PDFillustrated in FIG. 6 is applied across memory cell 10. PDF 14 iscentered at a current value of “I1” Accordingly, the most likely currentvalue is “I1” although there is some probability that the current willbe higher or lower than “I1.”

PDF 14 may be associated with one of four different resistive states ofmemory element 28, namely the state of the four having the highestresistance and therefore the lowest current. PDFs 16, 18, and 20 areassociated respectively with the other three resistive states of memoryelement 28. By way of example, if memory element 28 is in the secondresistive state and the voltage is applied across memory cell 10, theresulting current may be centered at “I2” and may have PDF 16.Similarly, if memory element 28 is in the third resistive state, theresulting current may be centered at “I3” and have PDF 18 and if memoryelement 28 is in the fourth resistive state (the lease resistive state),the resulting current may be centered at “I4” and have PDF 20.

FIG. 7C illustrates a combination of PDFs 14, 16, 18, and 20 into asingle PDF based on an assumption that memory element 28 is equallylikely to be in any of the four resistive states. Note that the PDF ofFIG. 7C peaks at current values “I1,” “I2,” “I3,” and “I4.” FIG. 7C alsoillustrates four ranges 36, 38, 40, and 42 that are associatedrespectively with the four resistive states of memory element 28. Ranges36, 38, 40, and 42 may be used to determine in which state memoryelement 28 is configured. For example, after applying a read voltage tomemory cell 10 and measuring the resulting current, if the current iswithin range 36 it may be determined that memory element 28 isconfigured in the resistive state associated with range 36 and thereforememory cell 10 is storing a particular bit value (e.g., “00”) associatedwith the resistive state.

However, when comparing ranges 36, 38, 40, and 42 with PDFs 14, 16, 18,and 20, one may conclude that it is possible that if a current fallswithin range 36, memory element 28 may be configured in the secondresistive state instead of the first resistive state. For example, ifthe read voltage applied to memory cell 10 is low (on the left hand tailof the PDF of FIG. 6) and memory element 28 is configured in thesecond-most resistive state (the one corresponding to PDF 16), theresulting current may fall within range 36 rather than within range 38as might be expected. Accordingly, errors may occur when reading memorycell 10.

FIG. 8A illustrates another embodiment of a relationship betweenvoltages applied across a memory cell and currents conducted by thememory cell as a result of the voltages. As depicted in FIG. 8A, as thevoltage across the memory cell increases from zero, the currentincreases to a transition point 68. As the voltage increases beyond thevoltage of transition point 68, the current decreases to transitionpoint 78. Further, as the voltage increases beyond transition point 78,the current increases.

In some embodiments, the voltage-current relationship depicted in FIG.8A may result from diode 26. Diode 26 may be band-gap engineered (as wasdescribed above) to produce a voltage-current relationship havingcharacteristics illustrated in FIG. 8A. Other voltage-currentrelationships having transition points in addition to transition points68 and 78 are also possible.

In some embodiments, insulative material 34 of diode 26 may comprise astack of three different dielectric materials (e.g., materials 54, 56,and 58 of FIGS. 2-3) as was described above. The plurality of layers maybe band-gap engineered to have particular barrier heights. For example,layers may be arranged in order of increasing or decreasing barrierheight. As a result, diode 26 may have a voltage-current relationshipincluding two or more transition points as illustrated by FIG. 8A. Infact, if the number of layers is increased, the number of transitionpoints may increase in some embodiments.

Thus diode 26 (e.g., an embodiment of diode 26 having a plurality oflayers of dielectric materials as illustrated in FIG. 2), may conduct afirst current from a first electrode (material 32) to a second electrode(material 22) when a first voltage is applied across the first andsecond electrodes of diode 26. For example, the first voltage and firstcurrent may correspond to the voltage and current at transition point 68of FIG. 8A. Diode 26 may alternatively conduct a second current from thefirst electrode to the second electrode when a second voltage is appliedacross the first and second electrodes. The second voltage may have thesame polarity as the first voltage and may have a larger magnitude thanthe first voltage. However, the second current may be smaller than thefirst current. For example, the second voltage and second current maycorrespond to the voltage and current at transition point 78 of FIG. 8A.

Diode 26 may alternatively conduct a third current from the firstelectrode to the second electrode when a third voltage is applied acrossthe first and second electrodes. The third voltage may have the samepolarity as the first and second voltages and may have a largermagnitude than the first and second voltages. The third current may belarger than the second current (e.g., if the third voltage and thirdcurrent correspond to point 69 of FIG. 8A) and may also be larger thanthe first amount of current (e.g., if the third voltage and thirdcurrent correspond to point 71 of FIG. 8A).

In some embodiments, a voltage-current relationship of diode 26 mayinclude two transition points (not illustrated) in addition totransition points 68 and 78 so that diode 26 may conduct a fourthcurrent from the first electrode to the second electrode when a fourthvoltage is applied across the first and second electrodes. The fourthvoltage may have the same polarity as the first, second, and thirdvoltages and may have a larger magnitude than the first, second, andthird voltages. The fourth current may be less than the third amount ofcurrent but larger than the first and second amounts of current.

Furthermore, diode 26 may have a threshold voltage above which diode 26may conduct current and both the first voltage and the second voltagemay be higher than the threshold voltage.

It should also be noted that if the first voltage and first currentcorrespond to transition point 68 and if the voltage is increasedslightly above the first voltage, the resulting current will be lessthan the first current. Similarly, if the voltage is decreased slightlybelow the first voltage, the resulting current will also be less thanthe first current.

FIG. 8B illustrates four superimposed current probability densityfunctions 44, 46, 48, and 50. PDFs 44, 46, 48, and 50 may be associatedrespectively with the four resistive states of memory element 28described above. PDF 44 may represent the probability that memory cell10 will conduct a particular amount of current when a voltage (such as aread voltage) having the PDF illustrated in FIG. 6 is applied acrossmemory cell 10.

PDF 44 may be associated with one of four different resistive states ofmemory element 28, namely the state of the four having the highestresistance and therefore the lowest current. PDFs 46, 48, and 50 areassociated respectively with the other three resistive states of memoryelement 28. By way of example, if memory element 28 is in the secondresistive state and the voltage is applied across memory cell 10, theresulting current may be centered at “I2” and may have PDF 46.Similarly, if memory element 28 is in the third resistive state, theresulting current may be centered at “I3” and have PDF 48 and if memoryelement 28 is in the fourth resistive state (the lease resistive state),the resulting current may be centered at “I4” and have PDF 50.

The shapes of PDFs 44, 46, 48, and 50 are different from the shapes ofPDFs 14, 16, 18, and 20 even though both sets of PDFs describe currentsthat result from voltages having the PDF of FIG. 6. This difference inshapes is due to the voltage-current relationship of FIG. 8A imposed bydiode 26 when diode 26 is band-gap engineered to have transition pointssuch as transition points 68 and 78. Due to this voltage-currentrelationship, PDFs 44, 46, 48, and 50 may have less overlap with eachother than the amount of overlap between PDFs 14, 16, 18, and 20. Insome embodiments, PDFs 44, 46, 48, and 50 might not have any significantoverlap.

FIG. 8C illustrates the combination of PDFs 44, 46, 48, and 50 into asingle PDF based on the assumption that memory element 28 is equallylikely to be in any of the four resistive states. Note that the PDF ofFIG. 8C has deep valleys between the values “I1,” “I2,” “I3,” and “I4.”Ranges 36, 38, 40, and 42 are illustrated in FIG. 8C and are associatedrespectively with the four resistive states of memory element 28. Whencomparing ranges 36, 38, 40, and 42 with PDFs 44, 46, 48, and 50, onemay conclude that it is highly probably that if a current falls withinrange 38, memory element 28 is configured in the second resistive state,not the first resistive state since there is little or no overlap of PDF44 or PDF 48 onto region 38. Consequently, a diode having avoltage-current relationship similar to FIG. 8A may significantly reduceerrors associated with determining the resistive state of memory cell 10as compared with a diode having a voltage-current relationship similarto FIG. 7A.

In fact, in some embodiments it might not be practical to use more thantwo resistive states in memory cells having a voltage-currentrelationship similar to FIG. 7A because of the high probability thaterrors may occur. Consequently, such memory cells might be configured inone of two resistive states and store only one bit of information. Incontrast, if a memory cell has a voltage-current relationship similar toFIG. 8A, four memory states may be used enabling the memory cell tostore two bits of information.

FIG. 9 illustrates another voltage-current relationship of memory cell10. According to this relationship, the current resulting from a voltageof “V1” may be a local maximum. If the voltage is increased above “V1”or decreased below “V1” the current may decrease. Similarly, voltages“V2,” “V3,” and “V4” may be local maximums. The voltage-currentrelationship of FIG. 9 may result from diode 26 if diode 26 is band-gapengineered (using methods described above) to produce such avoltage-current relationship.

In some embodiments, memory cell 10 may be configured in a firstresistive state by applying a programming voltage having a value of “V1”across memory cell 10. In some embodiments, the programming voltage mayhave a PDF with a Gaussian distribution similar to the PDF of FIG. 6.However, a programming current conducted by memory cell 10 in responseto the programming voltage may have a tighter PDF (e.g., one with asmaller width and/or standard deviation) than the PDF of the programmingvoltage because diode 26 may impose the voltage-current relationship ofFIG. 9 onto the programming current. The programming current may altermemory element 28 so that memory element 28 is in the first resistivestate.

Similarly, memory cell 10 may be configured in a second, third, orfourth resistive state by applying a programming voltage of “V2,” “V3,”or “V4” respectively across memory cell 10. Thus, diode 26 may narrowthe range of likely programming currents as a result of thevoltage-current relationship of FIG. 9. Consequently, diode 26 mayenable four distinct ranges of programming currents to program memoryelement 28 in one of four distinct resistive states. In someembodiments, the four ranges of programming currents may besubstantially non-overlapping and thus the four distinct resistivestates may also be substantially non-overlapping.

By way of example, memory cell 10 may be in a first resistive stateselected from among the resistive states associated with ranges 36, 40,or 42 of FIG. 8C. A first programming voltage may be applied acrosswordline 24 and bitline 22 so that a first current flows through memoryelement 28 and diode 26. The first programming voltage may be withinrange 112 of FIG. 9, which extends from voltage “V2” to a transitionpoint above voltage “V2.” As a result of the first current, theresistive state of memory element 28 may change from the first resistivestate to a second resistive state associated with range 38 of FIG. 8C.

Subsequently, memory cell 10 may be re-programmed to be in the firstresistive state instead of the second resistive state. Then, a secondprogramming voltage may be applied across wordline 24 and bitline 22 sothat a second current flows through memory element 28 and diode 26. Thesecond programming voltage may be within range 112 and may be largerthan the first programming voltage. As a result of the second current,the resistive state of memory element 28 may change from the firstresistive state to the second resistive state. Despite the fact that thesecond programming voltage is larger than the first programming voltage,the second current may be smaller than the first current due to thevoltage-current relationship of diode 26 illustrated in FIG. 9.

Other devices besides diode 26 may have a voltage-current relationshipsimilar to the voltage-current relationship of FIG. 8A. For example, twodiodes connected to form a thyristor may have a voltage-currentrelationship having two or more transition points similar to thevoltage-current relationship of FIG. 8A.

The memory cells of FIGS. 4-5 have a single diode in each memory cell.In other embodiments, multiple diodes may be utilized in a single memorycell. For instance, a pair of diodes may be provided in back-to-backarrangement to provide silicon controlled rectifier (SCR) or thyristortype properties. FIG. 10 illustrates a memory cell comprising aback-to-back diode arrangement. In referring to FIG. 10, similarnumbering will be used as is used above in describing FIGS. 1-9, whereappropriate.

FIG. 10 shows a portion 70 of a semiconductor construction, and shows aschematic electrical diagram 72 adjacent the portion 70 to illustrateelectrical components comprised by the portion 70. The schematicelectrical diagram 72 shows that the portion comprises wordline 22,bitline 24, memory element 28, a first diode 74, and a second diode 76.The first and second diodes are adjacent one another, and inback-to-back orientation relative to one another.

Portion 70 comprises base 12, memory component 52 (containing bitline24, memory element 28, and conductive material 32), and the diodes 74and 76 between the memory component and the wordline. Diode 74 comprisesdiode dielectric materials 80, 82, and 84 and diode 76 comprises diodedielectric materials 90, 92, and 94. Dielectric materials 80, 82, 84,90, 92, and 94 may comprise any of the compositions discussed above fordiode dielectric materials.

A conductive material 98 is between the diodes, and bridges the diodeswith one another. The conductive material may comprise any suitablecomposition, and may, for example, comprise one or more metals (forinstance, one or more of tantalum, platinum, tungsten, aluminum, copper,and gold) and/or one or more metal-containing compositions (forinstance, metal silicide or metal nitride).

The combination of diodes 74 and 76 of FIG. 10 may have avoltage-current relationship having two or more transition points suchas the voltage-current relationships of FIG. 8A or FIG. 9. Accordingly,when reading a memory cell including the combination of diodes 74 and76, currents having PDFs similar in shape to those illustrated in FIG.8B may be conducted by the memory cell. As a result, it may be practicalfor the memory cell to be configured in one of four different resistivestates, enabling the memory cell to store two bits of information due tothe fact that the PDFs are non-overlapping or minimally overlapping aswas discussed above.

FIG. 11 is a schematic circuit diagram illustrating one embodiment of anarray of memory cells. As depicted in FIG. 11, each memory cell includesa memory element 28 and two diodes—diode 26 and diode 30.

Having two diodes in a memory cell may provide advantages over having asingle diode in a memory cell. For example, diode 26 may have avoltage-current relationship similar to FIG. 8A or FIG. 9 that enablesthe features and benefits described above in relation to FIGS. 6-9.However, in some embodiments, diode 26 may allow a leakage current whenreverse biased that may be large enough (e.g., between 0.01 uA and 0.5uA) to consume an undesirable amount of power or disturb neighboringmemory cells. In contrast, diode 30 may have a very low leakage current(e.g., a current in the picoampere or femtoampere range) when reversebiased but might have a voltage-current relationship similar to thevoltage-current relationship of FIG. 7. By using diode 26 and diode 30in series, the memory cell may have both voltage-current relationshipsimilar to FIGS. 8A or 9 and may have very low leakage current whenreverse biased.

FIG. 12 shows a portion 86 of a construction and shows a schematicelectrical diagram 87 adjacent portion 86 to illustrate electricalcomponents comprised by portion 86. Similar numbering is used inreferring to FIG. 12 as is used above in describing FIGS. 1-11 whereappropriate.

Portion 86 comprises base 12, bitline 22, diode 26, memory element 28and wordline 24, each of which have been discussed in detail above. Inthe embodiment illustrated by FIG. 12, diode 26 includes conductivematerial 100 instead of material 22. Material 100 may comprise anysuitable composition or combination of compositions including, forexample, aluminum, tungsten, molybdenum, platinum, nickel, tantalum,copper, titanium, tungsten silicide, or tantalum silicide and material32 may include a different one of aluminum, tungsten, molybdenum,platinum, nickel, tantalum, copper, titanium, tungsten silicide, ortantalum silicide. Material 100, in some embodiments, may have athickness of from about 2 nanometers to about 20 nanometers. As wasdiscussed above in relation to FIGS. 1-4, material 34 may include aplurality of layers of different dielectric materials.

In addition, portion 86 includes diode 30. In some embodiments, diode 30may include a p-doped silicon layer 102 and an n-doped silicon layer104. In some embodiments, if diode 30 includes p-doped silicon layer 102and n-doped silicon layer 104, material 22 may comprise aconductively-doped semiconductor (e.g., conductively-doped silicon).Diode 30 may be configured to inhibit current from flowing from bitline22 through memory element 28 to wordline 24 when a voltage is appliedacross bitline 22 and wordline 24 such that bitline 22 is at a higherpotential than wordline 24 (when the memory cell is reverse biased). Insome embodiments, diode 30 may be configured to have a very low leakagecurrent (e.g. less than 0.1 picoA) when reverse biased.

Another embodiment of a memory cell having two diodes is shown in FIG.13. FIG. 13 shows a portion 88 of a construction and shows a schematicelectrical diagram 89 adjacent portion 88 to illustrate electricalcomponents comprised by portion 88. Similar numbering is used inreferring to FIG. 12 as is used above in describing FIGS. 1-12 whereappropriate.

Portion 88 comprises base 12, bitline 22, diode 26, memory element 28and wordline 24, each of which have been discussed in detail above. Aswas discussed above in relation to FIGS. 1-4, diode 26 may include aplurality of layers of different dielectric materials.

In addition, portion 88 includes diode 30. In the depicted embodiment,diode diode 30 includes a semiconductor material 110 and a metalmaterial 108 in direct physical contact with semiconductor material 110that together form a Schottky diode. Metal material 108 may comprise anysuitable composition or combination of compositions including, forexample, tungsten, tantalum, titanium, molybdenum, copper, cobalt,platinum, palladium, nickel, and their silicides. Metal material 108 mayadditionally or alternatively include one or more of zirconium,manganese, zinc, indium, scandium, yittrium, and their oxides.Semiconductor material 110 may comprise amorphous silicon and/orcrystalline silicon. In some embodiments, semiconductor material 110 maybe undoped.

Schottky diode 30 may be configured to inhibit current from flowing frombitline 22 through memory element 28 to wordline 24 when a voltage isapplied across bitline 22 and wordline 24 such that bitline 22 is at ahigher potential than wordline 24 (when the memory cell is reversebiased). In some embodiments, diode 30 may be configured to have a verylow leakage current (e.g. less than 0.1 picoA) when reverse biased.

The embodiment of FIG. 13 may be fabricated as follows. Initially,material 22 may be formed over base 12. Material 22 may be patterned byutilizing photolithographic processing and one or more etches.Subsequently, material 34 may be formed over material 22. In someembodiments, material 34 may be deposited on material 22 and may bepatterned using photolithographic processing and one or more etches.Material 34 may be deposited with any suitable methodology, including,for example, atomic layer deposition (ALD). As was discussed above,material 34 may include a plurality of layers of different dielectricmaterials. In this case, the layers of material 34 may be individuallydeposited and patterned using photolithographic processing and one ormore etches.

In some embodiments, the methods used in forming material 34 may beselected so that the methods do not substantially change the dimensionsof material 22 or otherwise render material 22 inoperable as anelectrode of diode 26. For example, a maximum temperature used informing material 34 may be below a melting temperature of material 22 sothat material 22 does not change dimension or shape as a result of theformation of materials 34.

Subsequently, material 32 may be formed over material 34. Material 32may be patterned by utilizing photolithographic processing and one ormore etches. Material 32 may be undoped and the formation of material 32might not use a temperature higher than a melting temperature ofmaterial 22. Next, semiconductor material 110 may be formed overmaterial 32. For example, semiconductor material 110 may be deposited.Semiconductor material 110 may be patterned by utilizingphotolithographic processing and one or more etches. As was mentionedabove, semiconductor material 110 may be undoped since doping and/orannealing semiconductor material 110 might involve temperatures higherthan a melting temperature of material 22 or material 34.

Next, metal material 108 may be formed over semiconductor material 110.Metal material 108 may be deposited, for example, using ALD. Metalmaterial 108 may be patterned by utilizing photolithographic processingand one or more etches.

Memory element 28 is then formed over metal material 108. Memory element28 may be formed by patterning the memory element material utilizingphotolithographic processing and one or more etches.

Wordline material is then formed over memory element 28. A maximumtemperature used in forming wordline 24 and memory element 28 may bebelow a melting temperature of materials 108, 32, and 22 so that thesematerials do not change dimension or shape as a result of the formationof wordline 24 or memory element 28.

FIG. 13 illustrates diode 26 provided between the bitline 22 and diode30. In other configurations, diode 26 may be provided between memoryelement 28 and diode 30. The fabrication process utilized to form suchother configurations may be similar to that utilized to form theconfiguration of FIG. 13, except that diode 26 may be formed after diode30 rather than before diode 30. In yet other embodiments, theorientation of the wordline and bitline may be reversed (so that thewordline is under the bitline) and the diodes may be formed eitherbetween the wordline and the memory elements, or between the bitline andthe memory elements.

In some embodiments, two memory cells stacked vertically may share asingle bitline. Fabricating such vertically stacked memory cells may usefewer processing steps than if the two vertically stacked memory cellseach had a different bitline.

FIG. 14 shows a portion 96 of a construction and shows a schematicelectrical diagram 97 adjacent portion 96 to illustrate electricalcomponents comprised by portion 96. Similar numbering is used inreferring to FIG. 14 as is used above in describing FIGS. 1-13 whereappropriate.

Portion 96 comprises base 12, two wordlines 24, two memory elements 28,two diodes 26, and a bitline 22, each of which have been discussed indetail above. As was discussed above in relation to FIGS. 1-4, each ofdiodes 26 may include a plurality of layers of different dielectricmaterials.

The upper memory cell of portion 96 may be read or programmed byapplying a voltage across the upper wordline 24 and the bitline 22 sothat the upper wordline 24 is at a higher potential than bitline 22 andthe upper diode 26 is forward biased. In response to the voltage, acurrent may flow from the upper wordline 24 through the upper memorycell to bitline 22. The current may be used to read the upper memorycell or to program the upper memory cell. While reading or programmingthe upper memory cell, a voltage may be applied across the lower memorycell that reverse biases the lower diode 26 so that the current flowingthrough the upper memory cell is inhibited from flowing from bitline 22through the lower memory element 28 to the lower wordline 24 therebyreducing the possibility that a resistive state of the lower memory cellwill be altered while reading or programming the upper memory cell. Toreverse bias lower diode 26, a voltage may be applied across bitline 22and the lower wordline 24 so that bitline 22 is at a higher potentialthan lower wordline 24.

A similar process may be used when reading or programming the lowermemory cell of portion 96 whereby the upper diode 26 is reverse biasedwhile the lower diode 26 is forward biased allowing a current to flowthrough the lower memory cell. The current may be used to read orprogram the lower memory cell. Since the upper diode 26 is reversebiased, the upper diode may inhibit the current from flowing through theupper memory element 28 thereby reducing the possibility that aresistive state of the upper memory cell will be altered while readingor programming the lower memory cell.

The embodiment of FIG. 14 may be fabricated as follows. Initially,material 24 may be formed over base 12. Material 24 may be patterned byutilizing photolithographic processing and one or more etches. In someembodiments, material 24 may be deposited on base 12 using any suitablemethodology, including, for example, atomic layer deposition (ALD).

Memory element 28 is then formed over metal material 24. Memory element28 may be formed by patterning the memory element material utilizingphotolithographic processing and one or more etches. Subsequently,material 32 may be formed over material 34. Material 32 may be patternedby utilizing photolithographic processing and one or more etches.Material 34 may then be formed over material 32. As was discussed above,material 34 may include a plurality of layers of different dielectricmaterials. In this case, the layers of material 34 may be individuallydeposited and patterned using photolithographic processing and one ormore etches.

In some embodiments, the methods used in forming material 34 may beselected so that the methods do not substantially change the dimensionsof material 24 or otherwise render material 24 inoperable. For example,a maximum temperature used in forming material 34 may be below a meltingtemperature of material 24 so that material 24 does not change dimensionor shape as a result of the formation of material 34. Forming material34 without annealing may be advantageous because annealing may involveundesirably altering dimensions of material 24 due to high temperature.

Material 22 may subsequently be formed over material 34. Material 22 maybe patterned by utilizing photolithographic processing and one or moreetches. In some embodiments, material 22 may be deposited on material 34using any suitable methodology, including, for example, atomic layerdeposition (ALD). The methods of forming materials 34, 32, 28, and 24are then repeated to form the upper memory cell.

FIG. 14 illustrates diode 26 provided between the bitline 22 and memoryelement 28. In other configurations, diode 26 may be provided betweenmemory element 28 and wordline 24. The fabrication process utilized toform such other configurations may be similar to that utilized to formthe configuration of FIG. 14. Furthermore, in some embodiments, one orboth of the memory cells of portion 96 may include two diodes (e.g.,diode 26 and diode 30) rather than a single diode as was described abovein relation to FIG. 13.

In compliance with the statute, the subject matter disclosed herein hasbeen described in language more or less specific as to structural andmethodical features. It is to be understood, however, that the claimsare not limited to the specific features shown and described, since themeans herein disclosed comprise example embodiments. The claims are thusto be afforded full scope as literally worded, and to be appropriatelyinterpreted in accordance with the doctrine of equivalents.

The invention claimed is:
 1. A current conducting device comprising: afirst electrode; a second electrode; dielectric material in physicalcontact with the first electrode and the second electrode; and whereinthe device is configured to conduct a first current from the firstelectrode through the dielectric material to the second electroderesponsive to a first voltage being applied between the first electrodeand the second electrode and to conduct a second current from the firstelectrode through the dielectric material to the second electroderesponsive to a second voltage being applied between the first electrodeand the second electrode, the second current being smaller than thefirst current and the second voltage being larger than the firstvoltage.
 2. The device of claim 1 wherein both the first voltage and thesecond voltage are larger than a threshold voltage, the device beingconfigured to conduct current from the first electrode to the secondelectrode when a voltage larger than the threshold voltage is appliedbetween the first electrode and the second electrode.
 3. The device ofclaim 1 wherein the device is configured to conduct a third current fromthe first electrode through the dielectric material to the secondelectrode responsive to a third voltage being applied between the firstelectrode and the second electrode, the third voltage being larger thanthe second voltage and the third current being larger than both thefirst current and the second current.
 4. The device of claim 3 whereinthe device is further configured to conduct a fourth current from thefirst electrode through the dielectric material to the second electroderesponsive to a fourth voltage being applied between the first electrodeand the second electrode, the fourth voltage being larger than the thirdvoltage and the fourth current being smaller than the third current. 5.The device of claim 1 wherein the device is configured to inhibitcurrent from flowing from the second electrode through the dielectricmaterial to the first electrode when a third voltage having a polarityopposite that of a polarity of the first voltage is applied across thefirst electrode and the second electrode.
 6. The device of claim 1wherein the first voltage and the second voltage both have a samepolarity.
 7. The device of claim 1 wherein the first electrode comprisesa first metal and the second electrode comprises a second metaldifferent than the first metal.
 8. The device of claim 7 wherein a firstwork function associated with the first metal is related to an amount ofenergy used to remove an electron from the first metal and a second workfunction associated with the second metal is related to an amount ofenergy used to remove an electron from the second metal, the first workfunction being lower than the second work function.
 9. The device ofclaim 7 wherein the dielectric material has a barrier height greaterthan both a work function of the first metal and a work function of thesecond metal, the barrier height being related to an energy differencebetween a conduction band of the dielectric material and a valence bandof the dielectric material.
 10. The device of claim 7 wherein the firstmetal is one of aluminum, tungsten, molybdenum, platinum, nickel,tantalum, copper, titanium, tungsten silicide, or tantalum silicide andthe second metal is a different one of aluminum, tungsten, molybdenum,platinum, nickel, tantalum, copper, titanium, tungsten silicide, ortantalum silicide.
 11. The device of claim 1 wherein the dielectricmaterial comprises at least one material selected from among silicondioxide, silicon nitride, titanium oxide, tantalum oxide, aluminumoxide, aluminum nitride, hafnium oxide, zirconium oxide, magnesiumoxide, yittrium oxide, and niobium oxide.
 12. The device of claim 1wherein the dielectric material comprises a plurality of layers, eachlayer of the plurality comprising a different dielectric material. 13.The device of claim 12 wherein the dielectric material is configured toform quantum wells at junctions between the layers of the pluralityresponsive to the first voltage being applied across the first electrodeand the second electrode.
 14. The device of claim 12 wherein each layerof the plurality has a different barrier height, each barrier heightbeing related to an energy difference between a conduction band and avalence band of one of the layers of the plurality.
 15. The device ofclaim 14 wherein the plurality of layers comprises a first plurality oflayers and wherein the dielectric material further comprises a secondplurality of layers of insulating material interposed between thedielectric material layers of the first plurality, the second pluralityof thin layers being configured to inhibit the first plurality of layersof different dielectric materials from mixing with one another.
 16. Thedevice of claim 14 wherein the plurality of layers are physicallyarranged between the second electrode and the first electrode in orderof increasing barrier height, a layer of the plurality nearest thesecond electrode having the lowest barrier height of the plurality and alayer of the plurality nearest the first electrode having the highestbarrier height of the plurality.
 17. The device of claim 12 wherein eachlayer of the plurality has a valence band energy level and the valenceband energy levels of the layers of the plurality are aligned.
 18. Thedevice of claim 12 wherein each layer of the plurality has a conductionband energy level and the conduction band energy levels of the layers ofthe plurality are aligned.
 19. A construction comprising: a substrate; asecond metal layer over the substrate; dielectric material over thesecond metal layer; a first metal layer over the dielectric material;and wherein the dielectric material is configured to conduct a firstcurrent responsive to a first voltage being applied across the firstmetal layer and the second metal layer and to conduct a second currentresponsive to a second voltage being applied across the first metallayer and the second metal layer, the second current being smaller thanthe first current and the second voltage being higher than the firstvoltage.
 20. The construction of claim 19 wherein the dielectricmaterial is in direct physical contact with both the first metal layerand the second metal layer.
 21. The construction of claim 19 wherein thedielectric material comprises a plurality of layers, each layer of theplurality comprising a different dielectric material, the dielectricmaterial being configured to form quantum wells at junctions between thelayers of the plurality responsive to the first voltage being applied.22. The construction of claim 21 wherein each layer of the plurality oflayers has a thickness less than or equal to 6 nanometers.